Thunderstorms are small scale weather patterns resulting from the formation of cumulonimbus clouds. The storms form as air currents move upwards into the atmosphere through convection. Overall, a thunderstorm usual builds and diminishes within about 30 minutes on average.
Thunderstorms, such as supercell storms, form when warm and moist air, atmospheric instability, and uplift combine to create storm clouds. Contrary to what you may think, not all thunderstorms are considered severe. In order to be considered a severe storm, the thunderstorm must produce some hazardous conditions. For instance, frequent lightning, high winds, straight-line winds, and large hail are hallmarks of a severe thunderstorm.
The National Weather Service does not consider a storm severe unless there is hail of at least 1.9cm (.75 in), tornadoes, and/or winds in excess of 93 km/hr (58 mi/hr). One point to remember is that the higher the top of a cumulonimbus cloud, the more severe the storm. In addition, although tornadoes are associated with thunderstorms, less than 1% of thunderstorms produce a tornado.